1. Field
Embodiments described herein relate generally to a non-volatile semiconductor storage device (EEPROM) including electrically rewritable non-volatile memory cells.
2. Description of the Related Art
With recent advances in miniaturization of NAND-type flash memory, reliability of data read becomes an issue.
NAND-type flash memory has memory cells connected in series, a source line connected to one end of each of the memory cells connected in series, and bit lines connected to the other end thereof.
When a data write operation is performed in the NAND-type flash memory, the voltage to be applied to memory cells will be changed in the following order. That is, a pass voltage Vpass (on the order of 8 V), which is as high as not to change a threshold voltage, is applied to the gates of unselected memory cells. As a result, the unselected memory cells are turned on. Then, a program voltage Vprg (20 V or more) is applied to the gate of a selected memory cell. As a result, electric charges are accumulated in the floating gate of the selected memory cell. Consequently, the threshold voltage of the selected memory cell increases (“0” data is written to the selected memory cell). This is a writing procedure.
In addition, a so-called sequential writing scheme is commonly employed for writing data to memory cells sequentially from the one closest to a source line.